The present invention relates to methods and apparatus for treating downhole formations in oil and gas wells and the like, such as for the hydrofracturing of a well formation through the use of expandable packer assemblies positioned downhole in the well bore.
While a large number of oil and gas wells are drilled to a depth and completed below 2,000 feet, nevertheless, a very significant number of wells are still being drilled and completed today at levels above 2,000 feet. Many of these wells are completed open-hole, without a well casing extending to considerable depths within the well bore, for reasons of enhanced primary oil or gas recovery and cost efficiency. The technique of hydrofracturing is currently used to make commercial wells out of marginal wells, yet the technological advances in hydrofracturing techniques have not significantly advanced or developed since prior to 1940. The present day known methods of hydrofracturing a section of a well bore formation downhole require the use of cable tool and completion rigs, which basically consist of a mast which must be erected at well site and held in position by stays, and a cable block and tackle assembly for lowering the packer assembly downhole on the end of a string of rigid pipes, with intermediate stops to connect additional pipe sections.
Presently, two basic methods of hydrofracturing are being utilized, both of which require the use of cable tool and completion rigs, in order to prepare a downhole zone within a well bore for hydrofracturing. The first method is to fill the well hole to a point just below the zone to be fractured with a fill, such as pea gravel. A single element steel and rubber packer assembly is then lowered into the hole on a string of rigid pipes, to a point just above the zone. The packer element on the packer assembly is then set or radially expanded to seal off the zone from the surface, and hydrofracturing equipment is then connected to the pipe string at the surface and the zone is hydrofractured by pumping a hydrofracturing fluid under pressure into the zone.
In order to hydrofracture lower additional zones in the well, the packer element is collapsed and a small diameter pipe string is lowered through the larger hydrofracturing supply pipe until the smaller coaxial inner pipe engages the fill. The fill is then washed from the annulus of the pipe to a point below the next zone within the well to be treated, and the packer assembly is then lowered and the process is repeated.
A second method commonly used in hydrofracturing a well involves the use of a two-element packer assembly, or a packer assembly having an upper and a lower expandable packing element (commonly referred to as a straddle or straddler packer), wherein the zone or section of the well formation to be treated is straddled or sealed off, and a ball dart is dropped in the packer assembly to plug the bottom of the packer. The packer assembly is positioned downhole and the packer elements are simultaneously set or expanded, sealing the annulus above and below the zone. The hydrofracturing equipment is connected to the upper end of the rigid pipe string above ground and the zone is fractured with fluid under pressure. Additional zones within the well are fractured by collapsing the packer assembly packing elements and moving the assembly to straddle the next zone and repeating the process.
Both of the above described methods require a considerable amount of time in operation, for example, approximately three hours or more in treating a 1,000 ft. well, and this assumes that no other problems are encountered, such as the packing elements of the well packer assembly becoming stuck in formation notches downhole.
This explanation of prior art techniques of hydrofracturing has been somewhat simplified, in that most well formations in the eastern United States consist of naturally cemented sand grains, and thus are extremely hard. Both the single element and straddle packers have pressure limitations necessitating the use, prior to fracturing, of notching the well formation, whereby the zones to be fractured are first subjected to a high pressure sand-laden stream of air or water thereby cutting a notch into the formation of the zone. The notch thus effects a plane of weakness aiding in the initial breakdown of the formation when subjected to fracture pressures. Use of such formation notches necessitates accurate measurement and placement of the straddle packer, because a packer element which is set or expanded in such a notch and subsequently pressured with hydrofracturing fluid, will often become permanently stuck in the well hole.
The prior art methods and apparatus utilized for fracturing a well formation are highly susceptible to the encountering of a number of problems when used for open hole completion. For example, the time required to erect or assemble and subsequently disassemble the cable tool and completion rig is considerable. The time element required for connecting and running pipe sections is also considerable. Leaks at the pipe joints are also possible due to frequent useage and connecting and disconnecting. It is extremely difficult to accurately determine the packer location in the well hole or to set the packer assembly opposite the producing formation to be treated.
Many of the prior art packer assemblies also require the use of metal slips and/or metal springs which commonly break, and the rubber or elastic sealing elements have been known to come off the packer assembly under pressure. On occasion, sand may also find its way under the resilient packer elements or sleeve, thereby preventing the packer from collapsing. In addition, it is not always possible to determine when the packer is, in fact, set or collapsed in the well hole.
Considerable time is also required to release and reset a packer assembly of the prior art, change the downhole position thereof, and disconnect and reconnect the hydrofracturing equipment from and to the pipe string each time a different zone within the well bore is to be treated. In addition, with some prior art packer assemblies, the outside diameter thereof is so close to the diameter of the well bore that difficulties are encountered in getting the packer down the well hole. Also, those packer assemblies which require the use of a ball dart as previously explained, sometime encounter the problem that the ball dart does not properly set in the bottom of the packer. The prior art packers which utilize metal slips also sometime encounter the problem that the metal slips won't hold in the formation to set the packer. The metal slips are generally first set to temporarily hold the packer and then the packer elements are thereafter expanded. The metal slips may initially not hold, or they may slip when the packer elements are expanded, causing the packer assembly to be improperly positioned.
It is a principal object of the present invention to provide a method and apparatus for treating well formations which are devoid of the aforedescribed disadvantages of the prior art.